The present invention relates to a method of erasing nonvolatile memory devices.
Recently, there has been an increasing demand for nonvolatile memory devices, which can be programmed and erased electrically and do not need a refresh function of rewriting data at specific intervals.
A nonvolatile memory cell is an element that enables electrical program/erase operations. The program/erase operations are performed by changing the threshold voltage of the cell as electrons are moved by a strong electric field applied to a thin oxide layer.
In the erase operation of the nonvolatile memory device, there have been significant advancements. In the case in which only a single level cell program operation was performed, the influence of distributions of an erased cell on the program operation was not significant. However, as multi-level cell program methods are commercialized, one memory cell may be represented by several states. In order to ensure a margin of each state, efforts for narrowing the distributions have been made. This is described below in connection with the erase operation. As the distributions when the memory cells are erased become narrow, distributions after corresponding cells are programmed become narrow. Accordingly, it is necessary to improve the erase operation to narrow the cell distributions to the greatest extent after the erase operation is carried out.